High quality lubricants should be, and generally are, paraffinic in nature, since paraffins have a high viscosity index. However, normal paraffins, in particular, are waxy in character, and contribute to a high pour point in the oil. Accordingly, waxy paraffinic feeds may be converted to lubricant base oils by hydroisomerization dewaxing, which creates branching on the paraffinic molecules. Hydroisomerization dewaxing typically produces a lubricant base oil having relatively high branching. While creating branching on waxy paraffinic molecules generally lowers the pour point, it also lowers the viscosity Index (VI). High amounts of branching are required with an all-hydroisomerization process to reach target pour and cloud points. Accordingly, the products resulting from hydroisomerization processes typically have less than optimal viscosity indexes due to the relatively high amount of branching. Lubricant base oil products produced by a hydroisomerization process may have branching characteristics similar to those products described in U.S. Pat. Nos. 6,096,940, 6,090,989, and 6,059,955.
Low pour points are desirable in lubricant base oils. A low pour point indicates that a lubricant base oil will flow and lubricate at low temperatures. Pour point is a measurement of the temperature at which the sample will begin to flow under carefully controlled conditions. Pour point may be determined as described in ASTM D 5950. Many commercial lubricant base oils have specifications for pour point. When lubricant base oils have low pour points, they also are likely to have other good low temperature properties, such as low cloud point, low cold filter plugging point, and low temperature cranking viscosity.
Lubricant base oils having pour-cloud point spreads below about 30° C. are also desirable. Higher pour-cloud point spreads require processing the lubricant base oil to very low pour points in order to meet cloud point specifications.
It is further desired to have lubricant base oils having high viscosity indexes. Viscosity Index (VI) is an empirical, unitless number indicating the effect of temperature change on the kinematic viscosity of the oil. Liquids change viscosity with temperature, becoming less viscous when heated; the higher the VI of an oil, the lower its tendency to change viscosity with temperature. High VI lubricants are needed wherever relatively constant viscosity is required at widely varying temperatures. For example, in an automobile, engine oil must flow freely enough to permit cold starting, but must be viscous enough after warm-up to provide full lubrication. VI may be determined as described in ASTM D 2270-93.
Pour point and VI can be linked to the branching on the paraffinic hydrocarbon molecules of the lubricant base oil. Creating branching on linear paraffinic hydrocarbons generally lowers the pour point and lowers the viscosity Index (VI). The VI tends to decrease sharply if the number of substitutions of equal length is doubled, but the pour point may be less affected. API Project 42 data (Research completed between Jul. 1, 1943 and Jul. 1, 1946 by American Petroleum Institute Research Project 42 at the Pennsylvania State College) showed that VI declined as branches were moved to the middle of the molecule for butyl, phenyl, and cyclohexyl branches on a linear paraffin.
Waxy hydrocarbons prepared from a Fischer Tropsch process are a good potential feedstock for preparing high quality lubricants. Advantageously, the Fischer Tropsch products contain little, if any, of typical petroleum contaminants, such as aromatic compounds, sulfur compounds, and nitrogen compounds. However, the initial Fischer Tropsch waxy paraffins are generally straight chain waxes. Accordingly, the Fischer Tropsch products need to be subjected to further processing or upgrading to provide high quality lubricant base oil stocks.
Many researchers have investigated ways of converting waxy feeds, in particular waxy feeds from Fischer-Tropsch synthesis processes, into lubricant base oil stocks. By way of example, in an attempt to create enough branching to lower the pour point but not excessive branching such that the VI is significantly decreased, prior art methods have used a combination of hydroisomerization with solvent dewaxing using amorphous or large pore zeolite catalysts (e.g. Beta zeolite) for the hydroisomerization step. Nonetheless, in prior art methods using this technique, considerable branching is still created.
By way of example, U.S. Pat. No. 6,090,989 discloses a hydrodewaxing process to make lubricant oil basestocks. The lubricant oil basestocks as disclosed therein contain paraffinic hydrocarbon components in which the extent of branching, as measured by the percentage of methyl hydrogens (BI), and the proximity of branching, as measured by the percentage of recurring methylene carbons which are four or more carbons removed from an end group or branch (CH2>4), are such that: (a) BI−0.5(CH2>4) of greater than 15 and (b) BI+0.85(CH2>4)<45. This calculation means that for a molecule containing 24 carbons, that molecule would have at least 2.5 branches per molecule, or for every 100 carbons there are greater than about 9 branches.
U.S. Pat. No. 6,008,164 discloses a method for producing a lubricant base stock from a Fischer Tropsch wax wherein the lubricant base stock has a preselected oxidative stability. The lubricant base oils are disclosed as containing a mixture of branched paraffins wherein the branched paraffins contain up to four alkyl branches and wherein the free carbon index (FCI) of the branched paraffins is at least about 3. The Examples of the '164 patent demonstrate lubricant base oils with 3.46, 3.14, 4.19, and 3.59 branches per molecule.
WO 99/45085 discloses an integrated process for preparing a lubricant oil base stock including an isomerization step followed by a solvent dewaxing step. In the process a waxy feed as disclosed therein, a waxy feed is isomerized to an intermediate pour point over a select molecular sieve and the isomerized oil is then solvent dewaxed. The lubricant oil base stocks obtained were disclosed as having viscosity indexes of greater than about 140. The Examples of the '085 publication demonstrate lubricant oil base stocks with viscosity indexes in the 140's with the highest being 156.
EP 0776959 A2 discloses a process for preparing lubricant base oils having a VI of at least 150 from a Fischer Tropsch wax feed comprising contacting the Fischer Tropsch wax feed with a hydroconversion catalyst under hydroconversion conditions; separating the hydroconverted effluent obtained into at least one lighter fraction and a heavy fraction; and dewaxing the heavy fraction to yield the base oil. The feed to the process is narrowly limited to a Fischer-Tropsch wax having a congealing point of at least 50° C. and has a boiling range where the difference between the 90% wt boiling point and the 10% wt boiling point is in the range of from 40 to 150° C. The hydroconversion catalysts are disclosed as being amorphous catalysts.
U.S. Pat. No. 6,096,940 discloses a process for producing a biodegradable hydrocarbon lubricant base oil. The process comprises contacting a 700° F.+ Fischer Tropsch wax feed with hydrogen over a bifunctional non-noble Group VIII metal catalyst to produce hydroisomerization and hydrocracking reactions at 700° F.+ conversion levels ranging from about 20 to about 50 percent on a once through basis, based on the weight of 700° F.+ feed converted to 700° F.− materials, to produce a C5-1050° F. crude fraction. The isoparaffins contained in the crude fraction are disclosed as having methyl branches in an amount of less than about 7.5 methyl branches per 100 carbons. From the C5-1050° F. fraction, a residual fraction having an initial boiling point ranging from about 650° F. to about 750° F. is recovered. The residual fraction is dewaxed and a dewaxed oil is recovered. From the dewaxed oil a biodegradable hydrocarbon base oil is recovered. In the Examples, the VI of the recovered lubricant base oil is in the 130's and 140's.
U.S. Pat. No. 5,059,299 discloses a process for maximizing the yield of lubricant oil base stocks having pour points of about −21° C. or lower and a viscosity index of about 130 and higher by the steps of 1) isomerizing the wax over an isomerization catalyst such that between about 15 to 30% unconverted wax remains in the oil fraction of the isomerate boiling in the lube boiling range, 2) fractionating the product, 3) solvent dewaxing the fraction boiling in the lube boiling range to a pour/filter delta T (the difference in temperature between the pour point of the dewaxed oil and the filter temperature) of 9° C. or less, and 4) recovering a dewaxed lube oil product. The dewaxing catalysts suitable for use in this invention are defined broadly and include catalysts such as fluorided alumina.
There has also been research into how to analyze the composition of lubricant base oils and how the properties of lubricant base oils are influenced by its composition. By way of example a publication by Kramer, D. C., et al, “Influence of Group II & III Base Oil Composition on VI and Oxidation Stability,” prepared for presentation at the 1999 AIChE Spring National Meeting in Houston, Mar. 16, 1999 teaches that field ionization mass spectrometry (FIMS) is especially valuable in determining the distribution of paraffins and naphthenes in Group II and III base oils. Below 1% aromatics, the authors found that the most effective way to further improve oxidation stability was to increase VI. In general, the authors found that the lower the concentration of polycyclic naphthenes in an oil, the higher its VI and its oxidation stability.
There remains a need for an efficient and economical process for converting waxy paraffinic feeds to high quality lubricant base oils, in particular lubricant base oils with good low temperature properties and high viscosity indexes.